The Global Positioning System (GPS) is a widely used satellite-based navigation system consisting of a network of satellites broadcasting pseudo-random noise (PRN) codes modulated on an L-band carrier (L1, L2). A GPS receiver uses measurements of the pseudo-random (PRN) code-phase and carrier-phase from four or more satellites to solve for the three-dimensional position of the receiver and to calibrate its internal time reference. The GPS receiver determines velocity from measurements of the carrier phase and doppler. Accuracy of the GPS solution is limited by the errors on the GPS signals and the geometry established by the positions of the satellites relative to the user.
For instance, there are areas of limited visibility of the sky where the user can observe and receive the satellite signals from only a limited number of the Satellite Positioning System (SATPS) satellites. Thus, in such an area it is impossible for the user to precisely solve for the three-dimensional position of its receiver, to calibrate its internal time reference, or to determine its velocity.
What is needed is a system of networked ground transmitters (GT) located in the well-known positions that together with the visible SATPS satellites and together with the base station positioned in the precisely known location allows the user to precisely determine its position location, its timing, and its velocity.